1. Field of the Invention
The present invention relates to a process for manufacturing a tyre. In particular, the present invention relates to a process for manufacturing a tyre comprising the steps of producing and assembling the tyre structural elements on a toroidal support and the step of precuring at least an inner surface of a green tyre by heating the toroidal support.
Furthermore, the present invention relates to a toroidal support to be used in a process for manufacturing a tyre.
2. Description of the Related Art
In the present description, the term “green tyre” is used to indicate the product which is obtained upon assembling tyre structural elements which include an elastomeric material in an uncured state.
Moreover, in the present description, the term “inner surface” of the tyre is used to indicate the innermost surface of the tyre which, when the tyre is cured and operatively mounted on a wheel rim, comes into contact with the inflating fluid of the tyre.
The tyre manufacturing process according to the present invention comprises the step of manufacturing a green tyre by consecutively producing and assembling together on a toroidal support the tyre structural elements. Such a manufacturing process is described, for instance, in the European Patent EP-928,680—in the name of the same Applicant—according to which the toroidal support is moved, preferably by a robotized system, between a plurality of work stations in each of which, through automated sequences, a particular building step of the tyre is carried out.
The manufacturing process further comprises the step of moulding the green tyre, so as to confer to the latter a desired geometrical conformation, and the step of curing the green tyre, so as to consolidate it. The step of moulding further confers to the green tyre a desired tread pattern.
The moulding and curing steps of the green tyre are carried out by introducing the latter into a moulding cavity defined within a vulcanization mould, whose inner shape matches the shape of the outer surface of the tyre to be obtained, and by introducing a pressurized fluid into a diffusion interspace (or diffusion gap) obtained between the inner surface of the green tyre and the outer surface of the toroidal support.
Such a tyre manufacturing process is described, for instance, in the European Patent EP-976,533 in the name of the same Applicant, according to which, during the pressing of the raw elastomer material against the inner walls of the moulding cavity, a radial expansion is imposed to the tyre by effect of the introduction into the diffusion gap of a pressurized fluid. The introduction of the pressurized fluid is preferably carried out by means of feeding channels (or ducts) formed in the toroidal support and terminating at the outer surface of the latter. During the fluid introduction, the tyre is sealingly engaged at its circumferential inner edges between the inner walls of the moulding cavity and the outer surface of the toroidal support, so as to delimit the diffusion gap at the circumferential inner edges of the tyre itself. Advantageously, the heat amount which is necessary for curing the green tyre is provided to the latter through the walls of the moulding cavity and by means of a heating fluid which is introduced into the diffusion gap. Preferably, the heating fluid is the pressurized fluid used for carrying out the pressing step or is at least part of said pressurized fluid.
Therefore, in accordance with this process, the manufacturing of a tyre is carried out in the absence of a vulcanization bladder which is commonly employed in conventional tyre manufacturing processes. The bladder is generally made of rubber and is inflated with a high-pressure heated fluid, e.g. steam, and inserted into the green tyre, enclosed in the moulding cavity, in order to press the tyre against the inner walls of the moulding cavity and to provide the tyre with the desired stable geometric conformation as a result of the cross-linking process to which the elastomer material forming the tyre is submitted.
However, in processes without a vulcanization bladder as described above, the pressurized fluid directly comes into contact with the inner surface of the green tyre, fact which may cause a plurality of inconveniences due to possible permeation of the fluid into the tyre structure not yet vulcanized. For instance, separations between adjacent elastomeric layers or strip-like elements, or between the elastomeric material and the metallic or textile reinforcing structures may occur, or even corrosion phenomena in the metallic reinforcing materials may be promoted.
In order to prevent possible permeation of the pressurized fluid into the green tyre, document EP-976,534—in the name of the same Applicant—describes a tyre manufacturing process comprising the step of associating at least one layer of precured elastomeric material with the inner surface of the green tyre. Said precured layer is suitable for obtaining a sufficient mechanical strength to diffusion and penetration of the pressurized fluid and, at the same time, a high fatigue strength, in particular during the moulding step of the tyre in order to avoid formation of fissures and cracks. Therefore, according to said document, the tyre manufacturing process comprises the step of forming at least one layer of raw elastomeric material on the outer surface of the toroidal support so that the successive manufacturing of the green tyre is carried out on the toroidal support carrying said layer of raw elastomeric material. Furthermore, the tyre manufacturing process comprises the step of precuring said layer before introducing the green tyre into the vulcanization mould, said precuring step being carried out by supplying heat to said layer through the toroidal support.
Preferably, the heating of the toroidal support is achieved thanks to the fact that the toroidal support comes from a previous vulcanization cycle or by means of infrared rays or equivalent means, such as electric resistors.
For example, document EP-1,075,929 discloses a process for manufacturing a tyre according to which a rigid toroidal support is used, the latter consisting of a plurality of sectors. According to said document, a portion of each sector is moulded in a thermally conductive material (e.g., an aluminum alloy) and incorporates an electrical resistance for providing heat to the green tyre during the curing step.
A further example is described in document JP 11-320,567 according to which each sector of a toroidal support is contacted, in correspondence of the inner surface thereof, with a circular segment provided with resistor heaters so that a heat amount is supplied to the outer surface of the toroidal support from the inner surface thereof for curing a green tyre.
The Applicant has observed that the step of precuring a layer of raw elastomeric material, commonly referred to as “liner”, i.e. the elastomeric layer which is suitable for ensuring the retention of the tyre inflating fluid, is particularly advantageous also during the conformation of the tyre since the precured liner is provided with high and uniform mechanical resistance.
This is particularly evident in the case the liner is obtained by winding an elastomeric strip on the outer surface of a toroidal support, as described, for instance, in the European Patent EP-928, 680 mentioned above. In that case, in fact, since the pressurized fluid exiting from the feeding channels provided within the toroidal support does not uniformly impact onto the liner surface, the fluid may cause dishomogeneities on the inner surface of the tyre and even a disuniform conformation of the tyre.
Moreover, the Applicant has observed that the tyre structural elements of elastomer material still in an uncured state, i.e. in a plastic state, when coming into contact with the pressurized fluid, may take an anomalous arrangement with respect to the design specifications.
In particular, the Applicant has noted that, especially at the very beginning of the moulding step, the carcass ply or plies may remarkably move from their expected positions in the bead regions due to the expansion to which the tyre is submitted by the pressurized fluid. Consequently, tensioning of the carcass ply or plies in the finished tyre—said tensioning being mainly determined by the moulding step—is inevitably lower than expected.
Furthermore, the Applicant has noticed that, especially in the early stages of the moulding step, the vulcanization pressure may give rise to phenomena of lack or accumulation of elastomeric material, in particular in the tyre bead regions. As a consequence of said phenomena, disuniformities and defects may occur in the tyre thereby causing the latter to be discarded.